Electronic systems are usually connected for data transmission by copper or other electrical conductors. Light guides, however, are increasingly being used in place of copper conductors for many applications. Because data exchange over light guides takes place using light beam pulses, which carry no electrical charges, data transmission over light guides is not sensitive to electromagnetic noise fields. Moreover, light guides are superior to copper conductors in terms of transmission rate and bandwidth.
A typical light guide, called a step-index fiber, includes a glass fiber enclosed in a jacket layer. Light is coupled out of the light guide by removing the jacket layer at a desired coupling point and by attaching a coupling element to the glass fiber core of the light guide. Although this coupling can take place at any point along the fiber, the coupling is permanent and fixed at that point.
Other methods for coupling light guides also exist. For example, in the field of integrated optical circuits, light guides in the form of glass layers are evaporated onto quartz glass substrates with lower indices of refraction than the glass layers. Prism couplers, which are prism-shaped dielectrics with higher indices of refraction than light guides, can also be used for coupling light into and out of light guides. Coupling by these methods is possible at any point on a light guide, but once the point is selected, the coupling is permanent and fixed at that point.
Because it is often necessary to transmit data to or from moving points; for example, to or from rotating or sliding data banks, machinery or cable drums. These known methods for coupling light guides are permanent and fixed, the known coupling methods cannot be used to transmit data to or from moving points. For that purpose, conventional electrical conductors with slip rings or slip rods are used. These are disadvantageous not only because of a lower bandwidth, but because of noise, electromagnetic interference, cost and bulk exhibited by such systems.